This invention relates generally to the field of semiconductor integrated circuits, and more specifically to communication among a collection of integrated circuits.
Integrated circuit chips ordinarily communicate with one another through external wiring. Typically, this wiring lies on a printed circuit board. In order to adapt the tiny dimensions of the integrated circuit to the larger dimensions of the wires on the printed circuit board, the integrated circuit is mounted in a xe2x80x9cpackagexe2x80x9d made of plastic or ceramic. The package is large enough for people to handle easily and also provides mechanical protection for the chip.
Integral to the package are a collection of metal conductors, one for each connection required on the integrated circuit. At one end, these conductors are large and physically strong enough to attach to the printed circuit board. At the other end, these conductors are of a scale similar to that of the integrated circuit. The actual connection between the integrated circuit and these package conductors is generally a gold or aluminum xe2x80x9cbonding wirexe2x80x9d that is welded to a pad on the integrated circuit at one end and to the small end of a package conductor at the other.
Thus, there are a series of conductors of varying size between one integrated circuit and the next. First, on the integrated circuit itself, a typical conductor leading from a circuit to the periphery of the integrated circuit is about one micron in width or less. Second, still on the integrated circuit, relatively large transistors drive a bonding pad on the periphery of the integrated circuit. Such bonding pads are about 100 microns square, a very large area when compared with other parts of the integrated circuit. Third, there is the bonding wire connected to the integrated circuit at the bonding pad. The bonding wire is typically 25 microns in diameter and 400 microns in length and provides the external connection to the bonding pad. Fourth, there is the conductor in the package that connects the bonding wire to the outside of the integrated circuit package. At its small end, it is slightly larger than the bonding pad. At its large end it is of a suitable scale for mounting the integrated circuit to a printed circuit board, typically about 500 microns in size, and on a center-to-center spacing of 1250 microns. Fifth, there is the wire on the printed circuit board. It is about 500 microns wide and typically on the order of a few centimeters in length. At the next chip, there is a similar set of conductors in reverse.
This elaborate arrangement of connectors from one chip to another has two drawbacks. First, it is costly. There are many parts involved and many assembly steps to put them together. The steps include making the packages, installing the integrated circuit chips in them, bonding the pads of the integrated circuit to the conductors in the package, and fastening the packages to the printed circuit board. Although each of these steps is highly automated, nevertheless they remain a major cost factor in many system designs.
Second, it is electrically undesirable. The wires on the printed circuit board are about 1000 times as large as the wires on the integrated circuit. Therefore, to send a signal from one integrated circuit to another requires a large amplifier on the sending integrated circuit. Moreover, the conductors involved have a good deal of electrical capacitance and electrical inductance, both of which limit the speed at which communication can take place. Perhaps worst of all, much energy is required to send a signal through such large conductors, which causes the driving integrated circuit to dissipate considerable power. The cooling mechanisms required to get rid of the resulting heat add cost and complexity to the system.
Several methods have evolved to improve chip to chip interconnect. One way is to avoid several packages for the separate integrated circuits. Instead of a package for each circuit, several chips are mounted in a xe2x80x9cmulti-chip module,xe2x80x9d a kind of communal package for the chips. The multi-chip module (MCM) contains wiring that carries some of the chip-to-chip communication circuits. The size of the wires in the MCM is smaller than the wires on a printed circuit, but not yet so small as the wiring on the chips themselves. Electrical capacitance and inductance in the wires between chips remains a problem even in MCMs.
An advancement made in the field of MCM fabrication is described in U.S. Pat. No. 5,767,009, illustrating in FIG. 18 of the patent stacked plural IC devices 91, 96. FIGS. 19A-19F of the ""009 patent illustrate the fabrication steps. First, as shown in FIG. 19A, a barrier metal layer 93 of titanium, palladium, or gold is formed by the electron beam evaporation method or the like. Then, the surface is covered with a photoresist 101 using photolithographic techniques, excluding an area of a first electrode pad 92, as shown in FIG. 19B. Then, in FIG. 19C, lead- or tin-based solder which is to become bump 95 is formed on barrier metal layer 93 above electrode pad 92 by means of electroplating or the like. After removing photoresist 101, barrier metal layer 93 is etched off with aqua regia, fluoric acid, or the like, leaving an area above the electrode pad in FIG. 19D. Barrier metal 98 is formed also on second semiconductor chip 96 by the same process. Next, as shown in FIG. 19E, bump 95 of first semiconductor chip 91 is aligned to barrier metal 93 of second semiconductor chip 96, and then the two are coupled together by heating or by pressing. Then, as shown in FIG. 19F, insulation resin 100 is provided between first semiconductor chip 91 and second semiconductor chip 96, and cured; thus the mounting of first semiconductor chip 91 on second semiconductor chip 96 is completed.
As can be seen, the fabrication of such multi-chip devices is not any less complicated than providing separately packaged IC devices and assembling the individual devices on a printed circuit board. The additional steps of laying down a metal layer and the various photolithographic steps increase the cost of manufacture and are a source of process problems which can lower production yields further adding to the overall cost.
An integrated circuit comprises first and second semiconductor dice. The first and second dice arranged so that their respective signal pads thereof are placed in face-to-face manner, forming lower and upper layers of semiconductor dice. Some of the signal pads of one die are in alignment with some of the signal pads of the other die. The first and second dice are spaced apart by air an gap, in one embodiment of the invention, and by a dielectric layer, in another embodiment of the invention. This arrangement creates capacitances between the aligned signal pad. Changing the electrical potential at a signal pad of the first die results in a corresponding electrical change at the opposing signal pad by virtue of the capacitive coupling. Signaling between the signal pads therefore is effectuated by detecting the changing electrical potential.
The semiconductor dice can be of a variety of shapes. The dice are arranged in planar fashion and in a regular pattern. In one embodiment of the invention, the upper and lower layer dice are rectangular in shape. In another embodiment of the invention, the dice have an octagonal shape.
A dielectric material is used to separate the first and second dice. In another variation, the signal pads are spaced apart by raised areas on the surface in which the signal pads are disposed. In yet another variation, the signal pads are spaced apart by recessing the signal pads below the surface of the semiconductor dice.